Many metal ions are essentially insoluble in aqueous systems having pH's ranging from approximately 6.0 to 9.0. Within this pH range, these metals, including but not limited to iron, zinc, aluminum and manganese, will deposit on heat transfer surfaces causing underdeposit attack.
Recirculating cooling water utilized in industrial applications may contain metal ions, such as iron and manganese which, upon deposition as insoluble salts on heat transfer surfaces, cause decreased operating efficiency and corrosive attack.
These concerns are particularly pertinent to recirculating cooling water systems employed by large utility plants.
Water treatment to prevent scale, corrosion and biological fouling in industrial and municipal aqueous systems is a complicated problem under the best of circumstances. Cooling water in utility plants, for example, may be particularly difficult to treat, especially when the source water contains high concentrations of iron and/or manganese. Also, such plants may be operated at several cycles of concentration. The resulting high levels of iron and manganese pose a challenge to conventional polymer treatment programs designed to stabilize these metals and prevent or inhibit deposits. The added problem of the presence of fouling microorganisms such as bacteria, particularly sulfate reducing bacteria, in the recirculating water generally mandates the use of an aggressive biological treatment program. Balancing the whole range of treatment needs makes it desirable that treatment additives not be antagonistic to one another. Of course, the treatment approach utilized must also take into account the extremes of plant operating parameters such as water quality, pH, cycles of concentration and temperature. All of these factors must be considered to determine the performance limits of any treatment additives utilized.
Polymers and phosphonates, alone and in combination, have been used in the past to stabilize deposit-forming metals ions such as Fe.sup.2+, Fe.sup.3+, and Mn.sup.2+. See for example U.S. Pat. Nos. 4,936,987, 4,640,793, 4,783,267 and 4,552,665. For many cooling towers, chlorine has been used as the biocide. Unfortunately, such biocides may inhibit the ability of polymer and/or phosphonate programs to stabilize metal ions. The instant inventors have surprisingly found that combinations of methylene bis(thiocyanate) and 2-(thiocyanomethylthio)benzothiazole, conventional biocides, enhance the ability of conventional water treatment polymers and/or phosphates to stabilize metal ions. This benefit of the instant biocide compositions is surprising and unexpected.
Hence, a long-felt need in the art of water treatment to find an effective biocide which does not antagonize metal ions stabilization by polymers and/or phosphonates has been met.
The use of methylene bis(thiocyanate) is well known in the art. See, for example, U.S. Pat. Nos. 4,612,328, 3,996,378, 3,300,375 and 3,524,871. However, its surprising effect on the ability of polymers and/or phosphonates to stabilize metal ions is not known or suggested in the art. Calgon product H-102 is a blend of 10% methylene bis(thiocyanate) and 10% 2-(thiocyanomethylthio)benzothiazole, on an active weight basis.